U.S. patent number 4,972,427 [Application Number 07/407,206] was granted by the patent office on 1990-11-20 for talbot cavity diode laser with uniform single-mode output.
This patent grant is currently assigned to Spectra Diode Laboratories, Inc.. Invention is credited to Donald R. Scifres, William Streifer, Robert G. Waarts, David F. Welch.
United States Patent |
4,972,427 |
Streifer , et al. |
November 20, 1990 |
**Please see images for:
( Certificate of Correction ) ** |
Talbot cavity diode laser with uniform single-mode output
Abstract
A diode laser of the type having an array of laser emitters in a
Talbot cavity in which edge reflectors are added to enhance
feedback to edgemost emitters. In one embodiment, a transparent
slab with reflectively coated sides is present between the phase
plane of the emitted light and the Talbot cavity reflector. The
phase plne is defined by a lenticular array placed a focal length
in front of the laser emitters. In another embodiment, the Talbot
cavity reflector has an increased reflectivity toward its edges. In
all embodiments the Talbot cavity reflector is preferably spaced a
distance na.sup.2 /.lambda. from the phase plane, where n is a
positive integer, a is separation between adjacent emitters and
.lambda. is the wavelength of emitted light. An integrated
embodiment has the array and cavity reflectors defined ina single
semiconductor body divided into active and ransparent region. Side
mirrors are etched into the semiconductor body. The laser array may
also be extended to two dimensions with individual lasers or laser
bars fiber coupled to a lens surface, with an edge reflector and
Talbot cavity reflector coated on an otherwise transparent
slab.
Inventors: |
Streifer; William (both Palo
Alto, CA), Waarts; Robert G. (both Palo Alto, CA), Welch;
David F. (San Jose, CA), Scifres; Donald R. (San Jose,
CA) |
Assignee: |
Spectra Diode Laboratories,
Inc. (San Jose, CA)
|
Family
ID: |
23611085 |
Appl.
No.: |
07/407,206 |
Filed: |
September 14, 1989 |
Current U.S.
Class: |
372/92;
372/50.12; 372/99 |
Current CPC
Class: |
H01S
5/148 (20130101); H01S 5/4062 (20130101); G02B
6/2813 (20130101) |
Current International
Class: |
H01S
5/40 (20060101); H01S 5/00 (20060101); H01S
5/14 (20060101); G02B 6/28 (20060101); H01S
003/05 () |
Field of
Search: |
;372/50,92,99,19,49 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4868839 |
September 1989 |
Simmons et al. |
|
Other References
James R. Leger, et al., "Coherent Addition of AlGaAs Lasers Using
Microlenses and Diffractive Coupling", Applied Physics Letters 52
(21), 23 May 1988. pp. 1771-1773. .
A. A. Golubentsev, et al., "Theory of Phase Locking of an Array of
Lasers", Sov. Phys. JETP 66, (4), Oct. 1987, pp. 676-682. .
John T. Winthrop, et al., "Theory of Fresnel Images. I. Plane
Periodic Objects in Monochromatic Light", Journal of the Optical
Society of America, vol. 55, No. 4, Apr. 1965, pp.
373-381..
|
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Schneck; Thomas
Claims
We claim:
1. A diode laser comprising,
an array of laser emitters, said emitters being substantially
identical and equally spaced apart at a light emitting plane;
a Talbot resonant cavity, said array of emitters being located
within said cavity; and
edge reflector means associated with said Talbot resonant cavity
for increasing feedback of light into edgemost ones of said
emitters.
2. The laser of claim 1 wherein a reflector of said resonant cavity
is spaced a distance of approximately na.sup.2 /.lambda. from a
phase plane of light emitted from said array, where n is a positive
integer, a is the substantially equal spacing between said emitters
and .lambda. is the wavelength of the light emitted from said array
in the region between said phase plane and said reflector.
3. The laser of claim 1 wherein said array is a linear array.
4. The laser of claim 1 wherein said array is a two dimensional
array.
5. The laser of claim 1 wherein said edge reflector means comprise
side mirrors extending from said array to said reflector and
laterally spaced a distance a/2 from said edgemost emitters.
6. The laser of claim 1 wherein said edge reflector means comprises
an increase of reflectivity of said reflector toward edges
thereof.
7. The laser of claim 1 wherein said Talbot resonant cavity is an
external cavity with at least one mirror distinct from a
semiconductor body forming said array.
8. The laser of claim 1 wherein said Talbot resonant cavity is
integral with a semiconductor body forming said array, said cavity
including reflectors defined by facets of said semiconductor
body.
9. The laser of claim 1 wherein a reflector of said resonant cavity
includes spatial filter means for enhancing feedback of a preferred
array mode relative to other array modes.
10. A diode laser comprising,
a semiconductor body having an active light emitting region with a
plurality of waveguides, said waveguides terminating in a light
emitting plane, said waveguides being substantially equally spaced
apart in said light emitting plane;
a pair of reflectors, a first one of said reflectors being defined
by a back face of said semiconductor body; and
edge reflector means associated with said pair of reflectors for
increasing feedback of emitted light into edgemost ones of said
waveguides.
11. The laser of claim 10 wherein a second one of said pair of
reflectors is spaced a distance of approximately na.sup.2 /.lambda.
from a phase plane of light emitted from said body, wherein n is a
positive integer, a is the separation between waveguides at said
light emitting plane and .lambda. is the wavelength of emitted
light between said phase plane and said second reflector.
12. The laser of claim 10 wherein said light emitting plane is an
antireflection coated front face of said semiconductor body.
13. The laser of claim 10 wherein said light emitting plane is a
boundary between said active light emitting region of said
semiconductor body and a transparent window region of said
semiconductor body, said second reflector being defined by a front
face of said semiconductor body adjacent to said window region.
14. The laser of claim 10 wherein said phase plane is defined by a
lenticular array spaced a focal distance in front of said light
emitting plane, said phase plane lying in the plane of said
lenticular array.
15. The laser of claim 10 wherein said edge reflector means
comprises side mirrors extending from said light emitting plane to
said second reflector and spaced a distance a/2 from edgemost
waveguides.
16. The laser of claim 10 wherein said edge reflector means
comprises an increase in reflectivity of said second reflector
toward its edges.
17. The laser of claim 10 wherein said second reflector includes
spatial filter means for enhancing feedback of a preferred array
mode relative to other array modes.
18. A laser comprising,
a laser bar having a reflective face, and an emitting face with a
plurality of substantially identical, equally spaced light emitting
elements;
a reflector being spaced from said emitting face by a distance of
approximately na2/.lambda., where n is a positive integer, a is the
spacing between said light emitting elements and .lambda. is the
wavelength of light emitted by said elements; and
a pair of side mirrors extending from said emitting face to said
reflector and spaced a distance a/2 beyond edge emitting
elements.
19. The laser of claim 18 further comprising a lenticular array
having a plurality of lens elements, one lens element per light
emitting element, each lens element being spaced between said
emitting face and said reflector a focal length away from a
corresponding light emitting element.
20. The laser of claim 18 wherein said reflector has low
reflectivity portions opposite from said light emitting elements
and higher reflectivity portions between said low reflectivity
portions across from areas of said emitting face between said light
emitting elements.
21. The laser of claim 18 wherein said pair of side mirrors
comprise a reflective material coating on side faces of a
transparent slab disposed between said emitting face and said
reflector.
22. The laser of claim 18 wherein said laser bar emitting face has
an antireflective coating thereon.
23. A laser comprising,
an array of substantially identical, equally spaced laser
emitters,
an array of lenses spaced a focal length away from said laser
emitters,
a partial reflector located at a distance of approximately na.sup.2
/.lambda. from the lens array, where "n" is a positive integer, "a"
is the center-to-center spacing between the laser emitters and
.lambda. is the wavelength of light emitted by the laser emitters,
said partial reflector having a variable reflectivity with higher
reflectivity at lateral edges thereof.
24. A laser comprising,
a semiconductor body having an active light emitting region with a
plurality of substantially identical, equally spaced waveguides,
and a transparent region bordering said light emitting region, said
waveguides terminating at a planar boundary between said light
emitting and transparent regions, said semiconductor body also
having a pair of reflective end facets, one said end facet being at
an end of said transparent region at a distance of about na.sup.2
/.lambda. from said planar boundary, where "n" is a positive
integer, "a" is the spacing between waveguides and .lambda. is the
wavelength of light in said transparent region; and
vertically oriented reflective side faces defined in said
semiconductor body on each lateral edge of said transparent region,
said side faces being spaced a distance a/2 from edge
waveguides.
25. The laser of claim 24 further comprising a vertical waveguide
defined in said transparent region, said vertical waveguide
extending from said planar boundary to said one end facet and from
side face to side face.
26. A laser comprising,
a plurality of diode lasers, each having a reflective back face and
a light emitting front face;
a plurality of light transmissive fibers, each fiber having a first
end coupled to a front face of one of said diode lasers, a second
end of said fibers terminating in a bundled plane, said second ends
being equally spaced; and
a transparent cylinder having a first end coupled to said second
ends of said fibers, a second end opposite from the first end with
a partially reflective coating thereon, and a highly reflective
circumferential surface.
27. The laser of claim 26 wherein said slab is cylindrical with a
highly reflective circumferential surface.
Description
DESCRIPTION
1. Technical Field
The present invention relates to semiconductor diode lasers and
monolithic integrated arrays of lasers, and in particular to lasers
having resonant cavities constructed for providing a
diffraction-limited, singlelobe far field beam.
2. Background Art
It is desirable to fabricate a high power semiconductor laser array
with good beam quality. Preferred is a beam with a coherent,
diffraction limited, single lobe far field, operating in a single
spatial and temporal mode, free from astigmatism with a low aspect
ratio. The laser should have a low threshold and high overall
efficiency and should be capable of being modulated at a high rate.
A laser which is also compact and robust is also advantageous.
Monolithic integrated arrays of semiconductor lasers or "laser
bars" typically have some but not all of the above noted features.
Some laser bars with closely spaced, phase-coupled waveguides are
capable of producing stable good quality beams, but are limited in
their power output due to problems with thermal dissipation. Other
laser bars are capable of power outputs greater than 50 watts CW,
but because the arrays of lasing elements are uncoupled, they tend
to produce a beam output quality which is characteristic of only a
single emitter.
One attempt to create a coherent diode laser array with higher
power has employed laser bars in an external Talbot cavity. Talbot
cavities rely on the phenomenon that an infinite one-dimensional
array of identical optical emitters with center-to-center spacing
"a" reimages itself at a distance Z.sub.T =2a.sup.2 /.lambda. or
any integer multiple thereof, where .lambda. is the wavelength of
the emitted light. Placing a reflector at one-half the Talbot
distance, Z.sub.T /2, or integral multiples thereof, causes the
emitters to reimage back upon themselves. Thus, this configuration
can serve as the basis for a laser cavity. The power from each
emitter couples into its neighbors by diffraction during the
propagation and reflection process, so the array locks coherently.
The waveguides in a laser bar need not be closely spaced anymore to
produce phase coupling, so higher power outputs can be produced.
In-phase emitters reimage on the reflector at distance Z.sub.T /2,
but laterally shifted by a/2. Thus, a partial reflector at Z.sub.T
/2 can function as the laser's output element and selectively
enhance the in-phase mode.
Even if Talbot reflectors are not placed at the planes defined
above, feedback for lasing in selected modes can still be obtained.
This may still represent a useful mode of operation. However, in
this case the reimaging is not perfect and will result in some
increase in threshold and loss in efficiency. This has been
described by J. Leger in Appl. Phys. Lett. 55(4), 24 July 1989,
pages 334-336.
Unfortunately, Talbot cavity lasers produced to date have not been
entirely successful. One problem has been poor mode discrimination.
While the preferred inphase array mode is dominant, it is not much
more so than the other array modes. Another problem results from
the fact that the laser bars are not infinite in length but have
only a finite number of emitters. Accordingly, the envelope of
model amplitudes is not uniform, but sinusoidal, so that
non-uniform gain saturation occurs. The residual gain allows other
modes to attain threshold. Thus, Talbot cavity lasers have not so
far achieved good beam quality.
An object of the invention is to provide a diode laser that
produces a coherent, high power diffraction limited beam with a
single lobe far field pattern.
Another object of the invention is to provide a diode laser in a
Talbot type cavity which has a uniform gain distribution and good
mode discrimination.
DISCLOSURE OF THE INVENTION
The above objects have been met with a diode laser having an array
of laser emitters in a Talbot resonant cavity in which edge
reflectors are added to the cavity. In a principal embodiment, the
edge reflectors are side mirrors that effectively image the finite
array to infinity, thereby causing the real laser to act like the
ideal model of an infinite number of identical emitters in a Talbot
cavity. Power from the diverging emitter light that was previously
lost at the edges is now reflected back toward the center and so is
available to be coupled back into the array. The array elements
thus have uniform gain in the preferred array mode. In a second
embodiment, the edge reflectors are defined by an increase in
reflectivity at or toward the edges of the Talbot reflector itself,
thereby increasing the feedback to the edge emitters. The Talbot
cavity may either be an external cavity or monolithically
integrated into the semiconductor body that forms the array of
laser emitters. The array can be a linear array or a
two-dimensional array of laser emitters fiber coupled into a
transparent cylindrical rod with the Talbot reflector on one
end.
Briefly, diode lasers of the present invention comprise a
monolithic array of light emitting elements, which generally are
defined by an active region with a plurality of waveguides in a
semiconductor body. The waveguides terminate in a light emitting
plane, which in the external cavity embodiments is an
antireflection coated face of the semiconductor body, and in the
integrated cavity embodiments is a planar boundary between active
and transparent regions of the semiconductor body. The lasers also
include a pair of reflectors, one of which usually being defined by
a back face of the semiconductor body and the other Talbot
reflector is spaced in front of the light emitting plane. For
optimum self-imaging and maximum reflection, the front Talbot
reflector is preferably placed at a distance of approximately
na.sup.2 /.lambda. from a phase plane of emitted light at or in the
space beyond the light emitting plane. In the latter case, a
lenticular array is spaced a focal length beyond the light emitting
plane, and the phase plane is in the plane of the lens array. Here
"n" is a positive integer, "a" is the separation between light
emitters, and .lambda. is the wavelength of the emitted light in
the space between the light emitting plane and the Talbot
reflector. As already noted above, the Talbot reflector includes
edge reflectors which can be side mirrors or increased edge
reflectivity. In order to further improve mode discrimination, the
Talbot reflector can also function as a spatial filter that
preferentially feeds back the desired array mode.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a first laser embodiment of the
present invention.
FIG. 2 is a top plan view of the laser in FIG. 1.
FIG. 3 is a side plan view of the laser in FIG. 1.
FIGS. 4 and 5 are graphs of array field amplitude versus lateral
position for Talbot cavity lasers of the prior art.
FIG. 6 is a top schematic illustrating the operating principle of
the laser in FIG. 1.
FIG. 7 is a graph of array field amplitude versus lateral position
for the laser in FIG. 1.
FIGS. 8 and 9 are partial top plan views of the laser in FIG. 1,
illustrating positioning of the lenticular array.
FIGS. 10a-c are graphs of components of far field intensity versus
angle for radiation from the laser in FIG. 1.
FIG. 11 is a top plan of a second laser embodiment of the present
invention.
FIG. 12 is a front end view of the laser in FIG. 11, illustrating
the reflectivity areas of the Talbot reflector.
FIG. 13 is a perspective view of the laser in FIG. 11.
FIGS. 14-16 are, respectively, top, side and front plan views of a
third laser embodiment of the present invention.
FIG. 17 is a top plan of a fourth laser embodiment of the present
invention.
FIGS. 18-20 are, respectively, partial enlarged side, top and front
end views of a fifth laser embodiment of the present invention.
FIGS. 21a-b are graphs of array field amplitude versus lateral
position, illustrating the concept of spatial filtering used in any
of the embodiments of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to FIGS. 1-3, a laser of the present invention
includes a monolithic linear array 11 of laser emitters 13,
hereafter referred to as a laser bar 11. Laser bar 11 has a high
reflectivity coating 15 on a back face and an antireflection
coating 17 on a front emitting face 16. The laser emitters 13 are
defined by waveguides 19 communicating optically with an active
light emitting region associated with a p-n junction 21. Waveguides
19 at the emitting face 16, and therefore laser emitters 13 are
spaced apart by a substantially equal distance a. The laser also
includes a lateral lenticular cylindrical lens array 23 and a
vertical cylindrical collection lens 25 positioned in front of the
emitters 13. Lens array 23 has one lens for each emitter 13, each
of width a, and is located a focal length f from the emitting face
16. Vertical lens 25 is closely spaced with or may be integral with
lens array 23. A glass slab 27 is disposed in the laser beyond the
lenses 23 and 25 and has side mirrors 29 and 30. A vertical
cylindrical reflector lens 31, having a front planar surface coated
with a pattern 33 of partially reflective stripes, is disposed in
front of the slab 27.
The laser bar 11 is preferably a monolithic array with ten or more
uncoupled single mode, real- refractive-index waveguide laser
elements 13. For example, the Spectra Diode Laboratories 5410 laser
bar has 100 mW cW lasers with emission spot radii approximately 0.8
.mu.m by 1.8 .mu.m (Gaussian beam waists) and divergences of
10.degree. laterally and 22.degree. vertically (full width half
maximum). The waveguided emitter elements are on 100 .mu.m centers
and are optically uncoupled. Coherent bars are not available. The
emitter outputs remain single mode up to approximately 150 mW and
are relatively insensitive to mode hops. The laser operates
reliably for over 10,000 hours without active cooling and has an
electrical-to-optical conversion efficiency of as much as 60%. The
manufacture of this and similar laser devices is fully described in
the literature of the art.
The laser bar 11 has facet coatings 15 and 17. Typically, back
coating 15 is a high reflection coating with over 95% power
reflectivity. The front emitting face coating 17 is an
anti-reflection coating with 2% or lower power reflectivity.
Materials, such as A1.sub.2 O.sub.3 and ZrO.sub.2, may be
deposited, and device characteristics monitored during deposition
to provide optimal coating thicknesses.
The laser bar 11 is mounted top down on a heatsink 35 for uniform
temperature. Minimizing temperature variations in the array are
important, because temperature differences may cause individual
emitters in the array to lase at different wavelengths, thresholds
and efficiencies. Preferably, the power dissipation of the heatsink
should be at least 1 W/mm. Active cooling of the heatsink 35 is not
required.
The lateral lenticular array 23 consists of individual lenses, one
for each emitter 13. Each lens in array 23 collects the light from
its corresponding emitter and collimates it in the lateral
direction. Alternatively, from the viewpoint of phase, each lens
converts the wavefront in the lateral direction, which has been
curved in propagating from the laser bar 11, into a planar front.
The lens array 23 should collect virtually all (at least 95%) of
the light emitted by each emitter 13 with a 10.degree.
divergence.
Several suitable fabrication techniques for lens array 23 are known
in the lens art, and their product is commercially available from
several sources. One type of lens array is a diffused lens type
which is fabricated by the electric-field-assisted exchanging of
sodium ions in a glass with diffused silver ions, through small
slits or openings in a mask, thereby increasing the index of
refraction in the neighborhood of the openings, producing graded
index (GRIN) lenses. Another lens array type is a plurality of
discrete lenses made by grinding, molding pressing, casting or
machining. A third lenticular array type employs binary optical
elements, such as holographic lenses or discrete Fresnel lenses.
Such arrays may be produced directly with a scanning e-beam or made
in quantity from a sub-micron photolithography defined mask, and
etching. Anti-reflection films may be deposited on discrete and
diffused lenses, and on the flat surface of binary lenses, but
cannot be coated on the relief surface of binary lenses without
distorting their behavior.
Referring to FIGS. 8 and 9, the lens array 23 should be positioned
to pass a planar phase front therethrough. If the focal length f of
the lens array is longer than the distance between the array and
the emitting phase 16, as in FIG. 8, then the diverging light 37
from emitters 13 will leave the lens array 23 with a curved
wavefront 39. After propagation to and from the reflector 33, the
phase fronts 41 would return to the lenses with the reverse
curvature. Under this condition, the light 43 passing through the
lens array 23 would only weakly couple into the waveguides 19. If,
however, the lenticular array 23 is located at approximately their
focal distance f to the emitters 13, as in FIG. 9, the beams 37
from the emitters 13 are collimated to generate a planar phase
front 45. Upon reflection, the light returning from the reflector
33 will also have a planar phase front. It will therefore be
efficiently coupled back into the waveguides 19 and the Talbot
cavity will be very efficient.
Referring again to FIGS. 1-3, the collection lens 25 and reflector
lens 31 are chosen together to produce a laser output 47 which is
stigmatic and has a 1:1 aspect ratio. The first requirement is met
when the wavefront at the reflector pattern 33 in the vertical
direction has a planar wavefront. The second requirement is met by
expanding the vertical width of the emitted light until the lateral
and vertical extent of the output light 47 are comparable. The
vertical cylindrical collection lens 25 is positioned so its focal
length is longer than its distance from emitters 13. Thus, the beam
49 continues to diverge after passing through lens 25. After
propagating over half the Talbot distance Z.sub.T /2=a.sup.2
/.lambda., it will have expanded to a full width of about 850-1000
.mu.m (at e.sup.-4 of peak intensity). The optical requirements of
lens 25 follow from Gaussian beam theory, which is well known in
the art. Focal lengths in the range from 200-400 .mu.m are typical
for collection lens 25, and distances from emitters 13 range from
150-350 .mu.m. The divergence results in a cylindrical phase front
at cylindrical reflector lens 31. Lens 31 collimates the light and
directs it at the partial reflector 33. Typically, focal lengths
for lens 31 range from 1.4 to 2.2 cm. If an integer multiple of
Z.sub.T /2 is used for the Talbot reflector distance, the focal
length of collector lens 25 must be increased accordingly (along
with that of reflector lens 31) to maintain a 1:1 aspect ratio and
a planar wavefront output.
Collection lens 25 is shown as a discrete element. Alternatively,
the lateral lens array 23 and vertical collection lens 25 may be
integrated into a single lens structure. Reflector lens 31 has a
back surface facing emitters 13 which is anti-reflection coated,
and a front output surface on which a partial reflector 33 is
coated. The partial reflector 33 may be formed in a pattern that
doubles as a spatial filter, as explained below with respect to
FIG. 21.
A slab 27 of transparent material, such as glass, is disposed
between collection lens 25 and reflector lens 31. Reflective
coatings on lateral sides slab 27 form side mirrors 29 and 30.
Referring to FIGS. and 5, the Talbot cavity model assumes that
there is an infinite array of emitters. If, however, the array 11
is composed of a finite number of emitters 13, the emitter pattern
will not be well produced at the reflector 33, half a Talbot
distance Z.sub.T /2=a.sup.2 /.lambda., or a multiple thereof, away
from the emitters, nor will the feedback into the array waveguides
19 be uniform. Again, "a" represents the separation between
adjacent emitters and .lambda. represents the wavelength of emitted
light in the space between the emitters 13 and partial Talbot
reflector 33. In FIG. 4, the field amplitude E.sub.1 (x) of emitted
light versus lateral position at the emitting plane 16 is
represented by curves 51. If the amplitude of each emitter 3 were
equal, as shown in FIG. 4, the resulting diffraction pattern
E.sub.2 (x) at one-half the Talbot distance Z.sub.T /2 would be
represented by the curves 53. The pattern of emitters 13 is not
well reproduced at the ends 55 where the images are weakened and
distorted. Further, upon reflection the feedback into the outer
waveguides 20 of the array 11 will be weaker than in the center. An
analysis of this situation provides a steady state solution in
which the array mode field amplitudes for N emitters is
approximately: ##EQU1## where the waveguides 19 are numbered in
order 1=1, . . . , N and is the array mode index. The field
amplitude for the lowest order mode =1 is seen in FIG. 5 and has a
sinusoidal envelope 57.
The above described problem illustrated in FIGS. 4 and 5 is
characteristic of previous Talbot cavity lasers which necessarily
have only a finite number of emitters 13. Because the envelope of
the modal amplitudes in the waveguides 19 is sinusoidal, the
injected charges do not recombine equally across the array and
non-uniform gain saturation occurs. This leaves residual gain for
higher order modes to achieve threshold. The side mirrors 29 and 30
introduced by the present invention solve this problem. Referring
to FIG. 6, light 59 from emitters 13 diverges laterally in both
directions, left and right, toward the center of the array and
toward an edge of the array. The light 59 interacts with light from
neighboring emitters 13, whereby, upon reflection, it couples to
waveguides adjacent to that from which it came. However, unlike
prior Talbot cavity lasers in which edge emitted edgewise directed
optical power was lost, side reflectors 29 and 30 of the present
invention redirects this edge directed light 61 back toward the
center of the array.
The effect of this redirection of optical power on the operation of
the laser can be found by viewing it as a reimaging of the array by
the side mirrors 29 and 30. Mirrors 29 and 30 are positioned a
distance a/2 to the right and left of the edgemost emitters 13,
i.e. one-half the emitter separation away, and perpendicular to the
emitting plane 16. Edgewise directed light 61 is reflected by
mirrors 29 and 30, and the reflected light 62 appears to come from
emitters 63 beyond the array. It is, thus, like an array of
waveguides 19 and 65 which extends to infinity, with emitters 13
and 63 producing light output 59, 61 and 67. While the waveguides
65 and emitters 63 are merely virtual images seen through
reflectors 29 and 30, the effect is the same as if they were real.
The resulting array mode field amplitudes with respect to lateral
position is seen in FIG. 7. The envelope 69 of the modal amplitudes
is a constant, thus residual gain is entirely used by the dominant
mode.
With reference to FIGS. 10a-d, the lateral radiation pattern of the
laser in the far field results from the coherent superposition of
the N radiators 47 from partial reflector 33. Because the array
field amplitude on the Talbot reflector 33 effectively images that
on the laser bar emitting plane 16, and is substantially constant,
the optical field distribution of each radiator 47 is identical.
This means that the far field radiation pattern can be separated
into two contributions. A first contribution is the radiation
pattern, i.e. Fourier transform, of any one of the identical
emitters. This defines the envelope function 71 in FIG. 10b. The
second contribution is the Fourier transform of an array of N
.delta.-functions, each of whose strengths is equal to the
corresponding complex amplitudes. This defines the array or
sampling function 73 in FIG. 10a. The overall far field pattern 75
in FIG. 10c is a product of the two contributions. The array
function 73 generally exhibits many distinct lobes spaced
approximately .lambda./a radians apart, each with a FWHM lobe width
on the order of .lambda./aN. For example if .lambda.=0.83 .mu.m,
a=100 .mu.m and N=10, then each lobe would have a width of about
0.05.degree. and would be separated from adjacent lobes in the
sampling function by 0.48.degree.. For a radiator beam waist of 35
.mu.m at the reflector 33, the envelope function 71 would have a
width (FWHM) of 0.25.degree. and has an intensity which is reduced
to 10% of its central value at 0.48.degree.. Accordingly, the
consequent radiation pattern 75 in FIG. 10c does not have
significant side lobes.
With reference to FIGS. 11-13, a second embodiment of the present
invention includes a laser bar 77 which is a semiconductor body
forming an array of laser light emitters 79. The array of emitters
79 is defined by an active region with a plurality of waveguides 81
in the semiconductor body. The waveguides 81 terminate in a light
emitting plane 83, which is one face of the semiconductor body. A
pair of reflectors 85 and 87 define a Talbot resonant cavity.
Reflector 85 is defined by the back face of the semiconductor body.
Talbot reflector 87 is preferably spaced about a distance nZ.sub.T
/2=na.sup.2 /.lambda. from a phase plane of the emitted light,
where Z.sub.T is the Talbot distance, n is a positive integer, a is
the substantially equal separation between adjacent emitters 79,
.lambda. is the wavelength of the emitted light in the space
between the phase plane and the reflector 87, and the phase plane
is defined by a lateral lenticular array 89. Lenticular array 89 is
spaced a focal length f beyond the light emitting plane 83 and the
phase plane lies in the plane of the array 89.
Like the first embodiment in FIGS. 1-3, the Talbot cavity of this
embodiment includes edge reflectors. However, these edge reflectors
91 are defined by an increase in reflectivity at or toward the
edges of the Talbot reflector 87 itself, rather than side mirrors
29 and 30. As seen in FIG. 12, the reflector 87 has a central area
93 of one reflectivity and lateral edge areas 91 of higher
reflectivity. Typically, central area 93 will have an average power
reflectivity in a range from 15 to 25% and an optical field
reflectivity p in a range from about 0.4 to about 0.5. To overcome
the optical power loss due to edgewise directed diffraction, the
edge areas 91 have an optical field reflectivity
.rho.'=.rho.(1+2.tau.) where .rho. is the central area's field
reflectivity and .tau. is the strength of nearest neighbor coupling
for the dominant mode. Typically, .tau. is in a range from 0.05 to
0.15.
With reference to FIGS. 14-16, a third embodiment of the present
invention monolithically integrates a Talbot cavity with side
mirrors into a semiconductor body. A semiconductor body 95 has an
active region 97, which is electrically pumped to generate light,
and a transparent window region 99, which is not pumped but has a
wider band gap than the active region so as to be nonabsorptive of
the generated light. A planar boundary 101 separates the active
region 97 from the window region 99. Active region 97 includes a
plurality of waveguides 103 that extend to boundary 101, thereby
defining light emitters 105. Window region 99 may include vertical
waveguides. Emitters 105 are equally spaced apart by a distance
"a", and are substantially identical. A pair of reflectors 107 and
109 are defined by the back and front faces of semiconductor body
95. Front reflector 109, adjacent to window region 99, is spaced a
distance na.sup.2 /.lambda. from boundary 101, where .lambda. is
the wavelength of light in the window region 99 and n is a positive
integer. Thus reflectors 107 and 109 define a Talbot resonant
cavity in which emitters 105 reimage on reflector 109. Side mirrors
111 and 113 extend from boundary 101 to front facet 109 and are
spaced a distance a/2 outward from the edgemost emitters 105. Side
mirrors 111 and 113 are formed by etching away regions 115 in the
window region 99 from the semiconductor body 95.
With reference to FIGS. 18-20, another laser diode embodiment of
the present invention uses a two-dimensional array of laser
emitters in the Talbot cavity instead of a linear array. The laser
diode includes a plurality of laser bars or individual diode laser
emitters 117 optically coupled to fiber waveguides 119. Typically,
waveguides 119 are physically connected at one end to the emitting
facets of laser emitters 117 and are bundled together in a
hexagonal close packed array at the opposite end. Fibers 119 allow
the laser bars or individual diodes 117 to be well separated for
better heat removal, while closely spacing the fiber outputs 120 at
the opposite end, Preferably, fiber waveguides 119 are polarization
preserving single mode fibers. Typically, such fibers have a 5 to 8
micrometer diameter core surrounded by a 125 micrometer diameter
cladding. The cladding diameter determines the minimum separation a
between nearest neighbors. In a hexagonal close packing, six fiber
emitters 120 are 125 micrometers from any given fiber output while
another six second nearest neighbors are about 217 micrometers from
that fiber output. In square matrix packing, an alternative
arrangement to the preferred hexagonal arrangement, four fiber
outputs are 125 micrometers from any given fiber output, four
second nearest neighbors are about 177 micrometers from that fiber
output and four third nearest neighbors are 250 micrometers from
that fiber output.
The plurality of laser emitters 117 lie in a Talbot resonant cavity
defined by a set of first mirrors 121 and a second Talbot mirror
123. Mirrors 121 are typically defined by the back facets of
emitters 117, while Talbot mirror 123 is spaced half the Talbot
distance Z.sub.t /2 or a multiple thereof from a phase plane 127 of
the emitted light. A lens 125 collimates the light from the outputs
120 of waveguides 119 to produce a planar wavefront at lens plane
127. Typically, waveguide outputs 120 are physically coupled to a
surface of the lens 125. An antireflection coating 133 covers the
lens surface. A transparent slab 129 of cylindrical shape, or
alternatively, of polygonal cross-section, occupies the space
between phase plane 127 and Talbot mirror 123. As noted above,
mirror 123 is a distance nZ.sub.T /2=na.sup.2 /.lambda. from plane
127. Here n is a positive integer, a is the separation between
nearest neighbors of light outputs 120 as imaged onto phase plane
127 by lens 125. .lambda. is the wavelength of the emitted light in
the slab material 129 between plane 127 and mirror 123.
In accord with the invention, the slab 129 has an edge surface 131
which is reflectively coated. The edge of lens 125 may also be
reflectively coated. Reflective surface 131 reflects the edgewise
directed diverging light from the edgemost emitters back toward the
center so that the feedback to the edgemost emitters is
substantially equal to that of the more central emitters.
Alternatively, the reflective surface 131 images the finite array
of emitters 120 to infinity, thereby effectively realizing the
Talbot cavity model. Each of the emitters radiates at substantially
equal amplitude in the dominant mode. Alternatively, the Talbot
mirror 123 could have variable reflectivity, as in the embodiment
in FIGS. 11-13, with the outer rim areas having higher reflectivity
than the central areas to compensate for the power lost from light
diverging out the sides.
With reference to FIGS. 21a and 21b, in order to further improve
the performance of the Talbot cavity lasers described above,
spatial filters may be added to the Talbot reflector to increase
mode discrimination. To illustrate the effect of a spatial filter
on a Talbot cavity laser, a laser bar 141 is shown in both FIGS.
21a and 21b, having a plurality of waveguides 143 defining
substantially identical, equally spaced emitters 145 in an emitting
plane 146. The field amplitude versus lateral position in the
emitting plane 146 is represented by curves 147 and 148 in the two
cases. Half a Talbot distance Z.sub.T /2=a.sup.2 /.lambda. in front
of emitting plane 146 is the Talbot reflector 149 and 150 in the
two cases. Again the field amplitude versus lateral position in the
reflector plane is represented by curves 151 and 152.
It is a property of Talbot cavities that if the emitters 145 all
radiate in phase (corresponding to the lowest order array mode), as
seen in FIG. 21a, then the image formed on the reflector 149 will
also be in phase, as shown by curves 151, albeit displaced
laterally by a/2, where a is the emitter separation. Likewise, if
the emitters 145 radiate so that nearest neighbors radiate
180.degree. out of phase (corresponding to the highest order array
mode), as seen in FIG. 21b, then the image will also be one of
alternating phase, as seen by curve 152, without the lateral
displacement. This pattern also holds true at odd multiples of the
half Talbot distance Z.sub.T /2.
This property can be exploited by placing scatterers, absorbers or
deflectors on the Talbot reflector 149 or 150 to discriminate
against the unwanted array mode. Alternatively, local areas of
increased reflectivity can also be placed on the reflector to
discriminate in favor of the desired mode. In FIG. 21a, areas of
lower reflectivity, represented by absorbers 155 are placed in
positions directly in front of the corresponding emitter positions
145 where the undesired highest order mode, represented by curve
152 in FIG. 21b, would have amplitude peaks. Areas of higher
reflectivity are located between filters 155 where the desired
lowest order mode has its peaks so as to enhance feedback of that
mode. Likewise, in FIG. 21b the highest order mode is favored by
placing absorbers 157 or other spatial filters in positions
displaced by a/2 from the corresponding emitter positions 145. This
type of reflector pattern is also seen in FIG. 1, where the pattern
33 has areas 34 of higher reflectivity displaced by a/2 from
corresponding positions of emitters 13, thereby favoring the lowest
order mode.
With reference to FIG. 17, a spatial filter can also be placed on
integrated Talbot cavity lasers by etching. A semiconductor body
151 has a plurality of waveguides 153 in an active region 155. The
waveguides 153 terminate at a planar boundary 158 between active
region 155 and a transparent window region 157. Back and front
facets 159 and 161 of the semiconductor body 151 serve as
reflectors. Facet 161 is spaced a distance of about nZ.sub.T
/2=na.sup.2 /.lambda. from the boundary 158, where n is a positive
integer, a is the waveguide separation and .lambda. is the
wavelength of emitted light in window region 157. Deflectors 163
are etched into front facet 161 at positions directly in front of
waveguides 153. These positions favor the in phase mode which
images between the deflectors 163. In an alternative integrated
embodiment the semiconductor body 151 is twice as long and includes
the portion 165 shown in phantom. Additional waveguides 167 are
added that terminate at an alternate mirror facet 169. Facet 169
replaces facet 161 in this alternate embodiment. The waveguides 167
also terminate at a boundary 171 between an active region 173 for
waveguides 167 and window region 157, now in the center of the
laser. The window region 157 has a length nZ.sub.T =2na.sup.2
/.lambda., instead of nZ.sub.T /2 from before. Deflectors,
scatterers or absorbers 163-are located in the center of window
region 157 along plane 161. Side mirrors 175 are etched into window
region 157 at a distance of a/2 from the edgemost waveguides.
The lasers described above all improve over prior Talbot cavity
lasers by including edge reflectors, in the form of side mirrors or
increased edge reflectivity, to maintain substantially equal gain
to all the laser emitters in the array.
* * * * *